fc3k: /api/admin tags prune-unused endpoint — Tier-A preview-then-commit flow

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/admin.py

@@ -179,3 +179,22 @@ async def tag_usage_count(tag_id: int):
             )
         )
     return jsonify({"count": count})
+
+
+@admin_bp.route("/tags/prune-unused", methods=["POST"])
+async def tags_prune_unused():
+    """Tier-A: dry-run preview list IS the prompt. UI calls with
+    dry_run=true first, shows the list, operator clicks button to
+    re-call with dry_run=false."""
+    from ..services.cleanup_service import prune_unused_tags
+
+    body = await request.get_json(silent=True) or {}
+    dry_run = bool(body.get("dry_run", False))
+
+    async with get_session() as session:
+        result = await session.run_sync(
+            lambda sync_sess: prune_unused_tags(
+                sync_sess, dry_run=dry_run,
+            )
+        )
+    return jsonify(result)